Physical Review X

Exposure of aqueous media to x rays has consequences in many areas, such as radiation damage in biological samples, radiation therapy, and corrosion. Ionizing radiation triggers photochemical processes leading to the formation of ions and radicals on a very short timescale. However, a precise description of the structural evolution of these newly formed species remains elusive. To that end, we image the structural dynamics of individual water molecules exposed to a short x-ray pulse in unprecedented detail.

Using a free-electron laser, we expose isolated water vapor molecules to x-ray pulses and record the momenta of the ensuing ions. A first x-ray pulse removes two electrons from the molecule, forcing a change in its structure; a second pulse removes two more electrons, triggering the molecule to fracture into two protons and a doubly ionized oxygen atom. The momenta of these ions preserve information on structural properties of the molecule after encountering the first x-ray pulse. We find significant molecular structural changes after core ionization and subsequent electronic relaxation, such as asymmetric deformation and bond-angle opening, leading to two-body or three-body fragmentation on a timescale of a few femtoseconds.

By following such dynamics step-by-step in real time, we thus record a molecular movie of a water molecule affected by ionizing radiation. The immediate response of a water molecule to x rays and the birth of ions and radicals sets the stage for subsequent radiation chemistry processes in a liquid environment.

The tunneling of a particle through a barrier is one of the most fascinating quantum phenomena. The motion taking place under the barrier, in a region forbidden by classical mechanics, is the subject of intense debate. Many experiments aim at measuring the time taken by the particle to go through the tunnel. Here, we take a completely different direction, revealing the influence of the dynamics under the barrier on the motion of the outgoing particle.

Our approach combines two key elements: the barrier is chiral—its structure cannot be superimposed on its mirror image—and it rotates in time. Specifically, our barrier holds the electrons inside a chiral molecule, set spinning by a photoionizing laser field whose polarization rotates. The electrons must pass through the spinning barrier of the molecule to escape. The clockwise or counterclockwise rotation of the molecule kicks the now-liberated electrons toward opposite directions. This chiral tunneling is like the rotation of a nut on a bolt, which translates the nut forward or backward depending on the handedness of the thread. By appropriately shaping the laser field, we produce quantum interference patterns that show that the electrons exiting a left- or right-handed barrier are phase shifted. The chiral motion within the barrier thus leaves its signature on both the amplitude and phase of the electron wave packets emerging from the tunnel.

Beyond an interest in chiral imaging, the imprint of the sub-barrier dynamics on the tunneling electrons will impact all forms of attosecond strong-field spectroscopies, notably photoelectron holography and high-harmonic generation.

Detection of the energy conversion pathways between photons, charge carriers (such as electrons), and a material’s atomic lattice is important to understand fundamental physics and to develop advanced materials and devices. Yet, such insight remains incomplete because of experimental challenges in disentangling the various signatures on overlapping timescales. Here, we show that a type of x-ray spectroscopy meets this challenge by providing an unambiguous and simultaneous view on the temporal evolution of the photon-carrier-phonon system.

In our experiments, we explore the potential of x-ray absorption near edge structure (XANES), a type of spectroscopy in which x-ray photons address electrons near the atomic core to interrogate electronic structure. We apply the method to graphite and reveal the entire flow of excitation across the photon-carrier-lattice systems. This capacity allows us to settle outstanding questions regarding the scattering mechanisms for electrons and holes and the ultrafast dissipation of energy into the lattice system via coherent nondisplacive phonon channels.

Our measurements establish the utility of core-level XANES with attosecond temporal resolution to achieve an unprecedented view on the temporal evolution of the photon-carrier-phonon system with surprising new results even for a seemingly well-studied system like graphite.

Ultrafast x-ray free-electron lasers enable detailed investigation of the structure and dynamics of materials with unprecedented temporal and spatial resolution. At the same time, there is growing interest in the use of ultrafast optical pulses to generate novel states of matter, with the goal of creating materials properties on demand. Here, we use ultrafast x-ray pulses to measure how the atomic lattice of tin selenide (SnSe) responds to ultrafast optical excitation, finding a novel photoinduced instability that drives the material toward a new high-symmetry structure.

SnSe represents a broader class of materials that provides a rich platform for tuning material properties by means of their multiple lattice instabilities. In our experiments, we excite a sample of SnSe with near-infrared laser pulses and use scattered x rays to observe coherent phonon dynamics. The time-resolved x-ray diffraction provides critical information on the atomic motion, including phase, from which we can quantify the atomic-scale structural rearrangements.

From the coherent phonon motion, we find that photoexcitation moves the structure further from the high-temperature phase toward a new structure. We identify this new structure as belonging to a higher symmetry space group that is distinct from any known phase of this material. First-principles calculations support these results and help us identify electronic states that play a decisive role in determining the photoexcited state.

Our results suggest strategies for stabilizing the higher symmetry structure as well as potentially controlling nontrivial topological properties. The results therefore have important implications for related materials in and out of equilibrium, as well as the broader goal of controlling materials’ properties with light.

The name of the game in modern technology is nanoscale integration. Yet, extreme ultraviolet (XUV) sources, which are increasingly instrumental to nanotechnology for fabrication and advanced metrology, remain bulky and expensive. Here we report on a functional chip-scale XUV device that merges methods from high-field photonics and nanotechnology to simultaneously generate and spatially control a coherent XUV beam.

We fabricate a structure on the surface of a magnesium oxide crystal. When illuminated by a femtosecond laser, it emits coherent radiation in the XUV range through a high harmonic generation process. The phase and amplitude profile, imprinted by the structure onto the generated light, leads to the rapid focusing of the light with unprecedentedly high numerical aperture of 0.35, and to its eventual convergence down to a radius of 150 nm.

In the future, shorter wavelength harmonics and higher numerical aperture structures will bring the size of the focal spot achieved with our technique down to the sub-100 nm scale. The unique combination of short wavelength, small focus, and high intensity, inherent to our method, will enable many applications, such as direct laser nanostructuring and nonlinear imaging with chemical specificity. In addition, our demonstration paves the way for chip-integrated XUV sources.